Mutation Research, 150 (1985) 333-345 333 Elsevier MTR 02028 Correspondence between effects of 5-azacytidine on SCE formation, cell cycling and DNA methylation in Chinese hamster cells Janet Shipley 1, Kazuo Sakai 1,2, Umadevi Tantravahi 1,2, Bernadette Fendrock 1,2 and Samuel A. Latt 1,2,3 Genetics Division and Mental Retardation Center I, The Children's Hospital, Boston, and the Departments of Pediatrics 2 and Genetics 3, Harvard Medical School, Boston, MA 02115 (U.S.A.) (Received 17 January 1985) (Accepted 18 January 1985) Summary The effects of 5-azacytidine (5-Aza-C), alone and in combination with mitomycin C, were measured on sister-chromatid exchange (SCE) formation and DNA methylation in different genomic regions of Chinese hamster ovary cells and in Chinese hamster ceils containing amplified, dihydrofolate reductase sequences and resistant to methotrexate. 5-Aza-C, when present for the penultimate preharvest cell cycle, induced SCEs in a manner consistent with a directly measured reduction in deoxycytosine methylation in cellular DNA. At higher 5-Aza-C concentrations, cell cycling was inhibited and both SCE induction and DNA demethylation tended to level off. Under appropriate conditions, 5-Aza-C also potentiated the induction of SCEs by mitomycin C. 5-Aza-C-induced DNA demethylation could also be detected in the vicinity of different DNA sequences with the use of comparative HpalI/MspI digestion, DNA blotting, and molecular probes. The efficiency of an individual demethylation event in inducing SCE induction appeared to be very low, compared with alkylating agents such as 8-methoxypsoralen, suggesting that SCE induction by 5-Aza-C might be an indirect effect from long range changes induced in cellular DNA or chromatin conformation. 5-Azacytidine (5-Aza-C), an agent known to inhibit the methylation of cytosine in DNA (Jones and Taylor, 1980), has a wide spectrum of effects on cellular differentiation (Taylor and Jones, 1982a) and gene expression (Razin and Riggs, 1980; Mohandas et al., 1981; Tantravahi et al., 1981; Jones et al., 1982; Sager and Kovac, 1982; Blasi and Toniolo, 1983; Cooper, 1983; Lieberman et al., 1983; Nakamura and Okada, 1983; Wolf and Migeon, 1983; Konieczny and Emerson, 1984; Correspondence to: Dr. Samuel A. Latt, Genetics Division, The Children's Hospital Boston, 300 Longwood Avenue, Bos- ton, MA 02115 (U.S.A.) Wolf et al., 1984; Yen et al., 1984). 5-Aza-C can alter chromosome condensation (Viegas-Pequignot and Dutrillaux, 1976, 1981; Schmid et al., 1984) and DNA replication (Sharer and Priest, 1984). It can also influence DNA conformation (Wang et al., 1979; Jovin et al., 1982; Felsenfeld et al., 1979) and DNA interchange, the latter assessed in terms of sister-chromatid exchange (SCE) formation (Banerjee and Benedict, 1979; Hori, 1983). 5-Aza-C can undergo a low level incorporation into cellular nucleic acids, although the associated inhibition of cytidine methyltransferase is dispro- portionately high compared with the actual amount of 5-Aza-C incorporated into cellular DNA (Taylor 0027-5107/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)